Spiro(indolino)benzoxazine or spiro(benzindolino)-benzoxazine photochromic compounds may be prepared by condensing a nitrosophenol compound with an indoline (Fischer's base) or indolium salt, e.g., iodide salt, as described, e.g., in U.S. Pat. Nos. 4,816,584, and 4,936,995.
In order to improve the yield of spiro(indolino) type photochromic compounds, e.g., of the type described in U.S. Pat. No. 4,816,584, the substituted or unsubstituted indoline or indolium salt should be reacted with nitrosophenol, e.g., 3,5-dimethoxy-2-nitrosophenol, containing as little as possible of isomers, e.g., 3-5-dimethoxy-4-nitrosophenol, that do not produce photochromic compounds.
However, the typically used methods of making, e.g., 3,5-dimethoxy-2-nitrosophenol, also may result in coproduction of substantial amounts, i.e., up to about 70 weight percent, and typically from about 35 to 50 weight percent of 3,5-dimethoxy-4-nitrosophenol, necessitating selective separation of the 2-nitrosophenol (ortho) isomer from admixture with the 4-nitrosophenol (para) isomer to obtain 2-nitrosophenol isomer of sufficient purity to produce acceptable yields of photochromic compound.
One such method of selectively separating the 2-nitrosophenol isomer from admixture with the 4-nitrosophenol isomer is described by Cronheim, J. Org. Chem., 12, 1, 7 & 20 (1947) wherein the 2-nitrosophenol isomer is selectively complexed with copper salts. However, attempts to duplicate this technique have not been successful.
In accordance with this invention, a straightforward, convenient, readily reproducible method has been found for preferentially dissolving 2-nitrosophenol (ortho) isomer from a mixture of 2-nitrosophenol and 4-nitrosophenol (para) isomers, which method enables isolation of 2-nitrosophenol isomer of at least 90 percent purity.